The present invention relates generally to disturbance compensation for orbiting spacecraft, and more particularly, to a feedforward control system and method that compensates for predictable disturbances that affect orbiting spacecraft.
When a spacecraft passes through the earth's shadow (known as an eclipse), it experiences a temperature change. A reduction in spacecraft temperature occurs upon entering the eclipse and a rise in spacecraft temperature occurs upon exiting the eclipse. This change in temperature affects all of the spacecraft components and appendages, such as antennas, for example. Some appendages, such as the solar wings, rapidly deform during this period. This phenomena, known as "thermal shock", causes errors in spacecraft pointing and, if left uncompensated, could disrupt services provided by communication systems on board the spacecraft.
The assignee of the present invention has produced numerous spacecraft in the past, and prior to the production of the Galaxy 601 model spacecraft, none of the spacecraft have compensation systems that explicitly compensate for eclipse thermal shock disturbances. In general eclipse disturbances are handled by a mode control system which is a closed loop control system that waits for a pointing transient to occur before compensation is applied.
Heretofore, thermal shock disturbances have been reduced by conventional closed loop methods. These methods require that a pointing error be sensed before any compensation can be applied. Also, the conventional closed loop schemes often require that the bandwidth of the system be raised to reduce the size of the pointing error due to eclipse thermal shock. The higher bandwidth has some negative side effects such as increased sensor noise transmission and reduced stability margins.
Accordingly, it is an objective of the present invention to provide a disturbance compensation system and method for use with a spacecraft that compensates for predictable disturbances exerted thereon. It is a further objective of the present invention to provide for a thermal shock compensation system and method that controls the spacecraft during an eclipse and reduces the effects of thermal shock thereon.